Implant back injection devices of the type with which the present invention is concerned conventionally comprise a main hollow body, secured to a hollow needle into which the implant to be injected is introduced. A secondary body, arranged coaxially inside the main body, surrounds the needle in which a piston rod is able to slide coaxially. The purpose of this piston rod is to ensure that the implant is deposited at the proper depth in the subject tissue. When this back injection device is pressed against the subject tissue, the main body starts to slide along the secondary body from a proximal position to a distal position to allow the needle to penetrate the subject tissue. The movement of the main body is accompanied by a simultaneous movement of the piston rod, which, when the needle is withdrawn from the subject tissue, remains fixed relative to the needle so as to allow the implant to be kept at the required depth in the subject tissue.
The method of use of such a back injection device is as follows. By holding the back injection device in one hand via its main body, the practitioner presses the distal end of the secondary body against the subject's skin. When the back injection device is suitably arranged, the practitioner pushes on the main body. Via the effect of this pressure, the main body starts to slide axially along the secondary body, allowing the needle, which is secured to said main body, to penetrate the subject's skin. At the same time, the main body drives the piston rod whose position relative to the needle and the implant therefore remains unchanged. It is at the moment that the needle has reached maximum penetration of the subject's tissue that the actual implant back injection operation occurs. Indeed, the practitioner, in a gesture not dissimilar to a conventional injection, will with his left hand if he is right handed, hold the back injection device against the subject's skin in order to minimise the movement of the needle and move his other hand so as to hold the secondary body against the skin via his thumb which presses on a button provided at the proximal end of the secondary body whereas, with his index finger and middle finger, he will control the return of the main body to its proximal position by using a finger rest fitted to said main body. During this gesture, the needle gradually exits the subject's tissue. The piston rod does not however accompany this movement of withdrawal by the main body. Indeed, retained by the secondary body, it is uncoupled from said main body and remains immobile, thus gradually penetrating the hollow needle as the latter exits the skin. The implant thus emerges from the needle, held in position at the correct depth in the skin via the distal end of the piston rod, which abuts against said implant. The piston rod is then uncoupled from the secondary body and coupled to the main body again such that the needle and the piston rod end the back injecting movement together in a position in which they are protected by the secondary body.
In course of use, it has been observed that the use of a back injection device of the type described above is not always convenient and is often misunderstood by practitioners. Indeed, a large number of practitioners think that the gesture is finished once the needle has penetrated as deep as possible into the subject tissue, omitting to carry out the back injecting operation which alone allows the implant to be deposited in the subject tissue. Other practitioners have attempted to carry out the back injecting gesture by placing their thumb on the button of the secondary body and passing their index and middle fingers under the finger rest as indicated. However, these practitioners hold the main body firmly in their other hand, which makes it impossible for the main body to return to its proximal position and thus for the needle to exit the subject tissue. More generally, it has been deemed inconvenient and painful for the subject to have to hold the back injection device in one hand to press it against the subject's skin and push in the needle, then to have to release said device in order to perform the back injecting gesture with the other hand.
It is an object of the present invention to overcome the aforementioned drawbacks in addition to others by providing an implant back injection device that simplifies as much as possible the implant injecting gesture under a subject's skin.